Phased array antenna system have extensive applications in the fields of radar, communications and electronic countermeasure systems, and the indispensable components include high-quality broad-band tunable microwave signal sources, broad-band radar baseband signal sources, and phase delay beam-forming network. With traditional electronic controlled phased array antennas, the radar baseband signal goes through amplification, power balance, and other processing after it is modulated into microwave signals. Then, it is phase-shifted via the placement of microwave phase shifters on the arrays or units. After changing the phase relationships among antenna units, it finally forms into a spatial microwave beam signal. However, on one hand, the traditional electronic controlled phased array radar suffers the “aperture effect” of the phased array radar, which limits its application in the fields of radar working bandwidth, high resolution measurement, radar imaging, and spread spectrum code. On the other hand, restrictions exist for the traditional phased array radar (including optically controlled ones), such as the limited range of tunable microwave signals, limited product of time and bandwidth of radar signals, and strong phase noise. At certain deviated frequency, bad phase noise characteristics result in that the returned signal is concealed in the phase noise of the carrier wave, and thus, the target may not be properly detected.
With the development of the microwave photonics technology and extensive applications in the radar field, restriction on aperture transit time in the traditional phased array antenna is effectively overcome by the application of the true-time delay line technology. Optically controlled wave beam forming network based on the true-time delay may be used for scanning antenna wave beams with the benefits of broader instantaneous bandwidth, no wave beam squinting effect, low loss, small dimension, anti-electromagnetic interference, and long detection distance. It has become an important direction for the development of the phased array radar. In the mean time, optoelectronic oscillators are advantageous in generating microwave signals with low phase noise and broad range of tunable frequency, and they have great potential in the applications in optical signal processing, radar, communications, and bio-imaging detection.
Optically controlled phased array radar in the current research mostly adopts the architectures similar to those in K. Garenaux et al., “Recent breakthroughs in RF photonics for radar systems,” Aerospace and Electronic Systems Magazine, IEEE, vol. 22, pp. 3-8, 2007. The transmission link works as follows: corresponding carrier signals and modulated signals generated by the microwave signal generator are electro-optically transformed via a modulator, pass through a delay network structure, are transformed from optical signals to electric signals via an optoelectronic detector, and then are transmitted via microwave T/R components and antennas. The receiving circuit, on the other hand, receives reflected signals via microwave antennas and T/R components, conducts frequency mixing, conducts phase compensation by reusing the delay network of the transmission line, and finally processes the signals.
For the series delay systems of the optically controlled phased array radar in the current technology, the delay scheme based on multi-wavelengths has broad delay range, no power cycle fading resulting from dispersion, and thus, its front end needs the laser arrays or multi-wavelength lasers. The laser arrays or multi-wavelength lasers may combine with the delay network to form a microwave photonic filter. It may be a finite response microwave photonic filter with relatively high Q factor when the number of the laser arrays or the wavelength number is large. By employing the microwave photonic filter in a close-loop, an optoelectronic oscillator with tunable bandwidth is made.